Traffic and other doppler radar systems have been known for many years. The typical traffic radar employs a high-frequency source of electrical energy which is transmitted as a beam in the direction of a vehicle and the reflected signal from the vehicle is passed through a detector and duplexer which produces an output audio signal. This signal is equal to the frequency difference between the transmitted and received signal. This frequency difference is representative of the speed of the vehicle in accordance with the well-known doppler principle. Typical of the prior art doppler radar systems is that disclosed in U.S. Pat. No. 3,438,031.
The basic principle upon which a stationary CW doppler radar operates is set forth in the following well-known equation: EQU F.sub.d =(2v/c f.sub.t
Where
F.sub.d is the doppler frequency
F.sub.T IS THE TRANSMITTED FREQUENCY
V IS THE TARGET VELOCITY
C IS THE SPEED OF LIGHT
This equation assumes that the vehicle is headed on a radial path directly toward or away from the transmitter. A typical nominal transmitter frequency used in radar of this kind is 10.525 .times. 10.sup.9 Hz and the doppler frequency F.sub.d in Hertz as a function of target velocity, v, in miles per hour is then: EQU F.sub.d (Hz) = 31.389 v (mph)
Thus, for speeds from zero to 400 miles per hour, the doppler signal F.sub.d is within the audio range of roughly 0 to 12 kHz. Throughout this specification the 10.525 .times. 10.sup.9 Hz transmitter frequency will be assumed in discussing speeds, time intervals, doppler frequency-speed analogs and the like.
Doppler radar systems of the type described herein generate an audio signal which is normally accompanied by substantial noise and spurious energy. Thus, the detection and measurement of the doppler signal is frequently difficult and subject to substantial error. The prior art radar has failed to fully solve the problem of noise and measurement accuracy with optimum sensitivity. Furthermore, the frequency range which must be covered is about 0 to 12 kHz and circuits capable of operating over this range have been generally complex and expensive with slow response times and poor noise immunity. The speed range under about five miles per hour, which, in the system described above, would have an analog frequency under about 157 Hz has been beyond the range of most known systems heretofore. For certain applications, such as railroad yard monitoring, it is desirable to measure speeds as low as 0.5 mph and measure doppler frequencies as low as 15 Hz.